Electronic switches used in electrical energy converters such as choppers or inverters are generally semiconductor components such as thyristors, transistors or gate turnoff (GTO) thyristors.
At present the highest voltage which semiconductor devices are capable of withstanding is a few hundreds of volts for transistors and a few thousands of volts for thyristors and GTO thyristors.
When the supply voltage for an electrical energy converter needs to be higher than the voltage which the chosen semiconductor devices can withstand, a plurality of such devices must be connected in series.
The preferred application of the present invention lies in connecting transistors or GTO thyristors in series.
There is a major drawback to connecting transistors or GTO thyristors in series. Manufacturing tolerances are rather wide for such devices, particularly concerning the time lapse between receiving an instruction to open the switch and the switch becming effectively open.
Thus, when two transistors or two turnoff thyristors are connected in series without taking any precautions, the faster acting device at turnoff is temporarily subjected to the full power supply voltage and is destroyed.
Various circuits exist for eliminating this drawback. In particular, the journal "G.E.C. Journal of Science and Technology", volume 47, N.degree. 3, year 1981, pages 121 and 122 describes one such circuit applicable to transistors, and Toschiba's "GTO Application Note," page 17 describes such a circuit applicable to GTO thyristors. In each case a capacitor is connected in parallel with the switching device.
However, these circuits still suffer from two other drawbacks.
Firstly the voltage is not exactly equalized between the two switches since the capacitor associated with the faster acting device at turnoff charges more than the capacitor associated with the slower turnoff device. As a result, both devices in any particular circuit design must have a higher specified maximum permissible voltage than would otherwise be the case.
Secondly, in order to keep this lack of equalization within tolerable limits, capacitors of relatively high capacitance must be used. The energy stored in the capacitors is thus relatively high, and since this energy is dissipated in resistors each time the switches are turned off, efficiency is reduced, and an electrical energy converter using such an arrangement needs extensive cooling.
An improvement may be obtained by using transistors or GTO thyristors which have been selected for matching turnoff characteristics, and only connecting closely matching devices in series. However, this complicates manufacture and thus increases the cost of the devices, and also complicates maintenance of energy converters made using such matched devices.
Preferred embodiments of the present invention improve equalization of voltages across transistors or GTO thyristors connected in series without requiring the devices to have matched characteristics, and with reduced energy losses compared to prior art circuits.